"The volume of the gas reduction is around 40 percent. Gas deliveries are being carried out through parallel gas pipelines," Gazprom said in a statement.
The blast destroyed one of the three pipelines used to supply natural gas to the Balkans.
Angel Semerdzhiev, Bulgartransgaz chief executive, said it usually took up to three days for repairs to be carried out, but he was hopeful that deliveries to Bulgaria and the Balkans would be restored "soon".
Bulgaria receives all of its gas from Russia via Ukraine, Moldova and Romania, and also sends it ons to neighbouring Turkey, Greece and Macedonia.
Several of those countries also reported supply shortages on Wednesday.
Semerdzhiev promised that Bulgarian consumers would not be affected, since Bulgartransgaz would use reserves from its underground storage facility in northwestern Bulgaria to compensate for reduced supplies.
He did not say if Bulgaria was still pushing gas through to the countries further down the transit route.
Gazprom said it would increase deliveries to Turkey through the Blue Stream link under the Black Sea to compensate for any problems.
Bulgaria, which lacks any access to alternative gas sources or routes, was among the countries worst affected by a row between Russia and Ukraine in January that left much of eastern Europe without supplies in the depths of winter.